1. Field of the Invention
The present invention relates to a breakdown voltage characteristic of a vertical bipolar device, and more particularly to a method for measuring a collector and emitter breakdown voltage of a bipolar transistor which can measure a punch through breakdown voltage characteristic between a collector and an emitter with high accuracy.
2. Prior Art
In a vertical bipolar device, a manufacturing margin for the depth of each junction of a collector, a base and an emitter has been strictly restricted with an increase in fineness and performance of the device, and it is one of important factors that a breakdown voltage characteristic between electrodes, particularly, a punch through breakdown voltage characteristic between the collector and the emitter should be accurately defined ("Very High Digital Device Series 1, Very High Bipolar Device", Chapter 2, section 3-5 (pages 56 to 60) by Baifukan, the first edition was issued on Nov. 15, 1985).
For this reason, usually, a method is employed in which a collector and an emitter are connected to a measuring terminal and a base is disconnected from the measuring terminal, a voltage source is connected to a collector connecting terminal and an emitter connecting terminal is connected to a ground, thereby applying a voltage between the collector and the emitter.
In this method, however, the voltage is applied between the collector and the emitter so that a reverse bias is greatly applied to a collector and base junction and a depletion layer extends onto the collector side. On the other hand, holes of some electron--hole pairs generated in the collector junction enter a base region. However, since the base is in a disconnected (open) state, a base current does not flow and a voltage drop is not caused. Therefore, the same electric potential as in the collector and base junction is applied to An emitter and base junction. Accordingly, even if the collector and base junction takes a voltage (collector and base breakdown voltage) for causing avalanche breakdown, punch through is not caused and the depletion layer does not reach the emitter and base junction. Therefore, the emitter and base junction breaks down as soon as the avalanche breakdown is caused. For this reason, it is apparent that the above-mentioned prior art has not taken sufficient countermeasures to cause no destroy of the device and to accurately perform measurement.
For example, Japanese Patent Application Laid-Open NO. Sho-62-86757 has disclosed a technique in which a resistive element and a diode element are connected in series between a base and an emitter of a device and a polarity thereof is reverse to that of a base and emitter junction, thereby extracting measuring terminals from both poles of the diode element.
In this technique, when a voltage in applied between the collector and the emitter, a voltage is applied to the diode in a reverse direction. Therefore, the base and emitter is set in an electric potential state which is nearly a disconnected (open) state. However, a reverse junction leakage current of the diode flows into a base region through a resistor between the base and the emitter so that a depletion layer can reach the emitter and base junction. For this reason, the prior art has a sort of effects of accurately measuring a punch through breakdown voltage between the collector and the emitter.
According to the technique described in the Japanese Patent Application Laid-Open NO. Sho-62-86757, however, when a backward voltage is applied between the emitter and the base, the diode element is set forward. Therefore, an emitter and base breakdown voltage (zener breakdown) cannot be accurately defined. Therefore, there has been another problem in that characteristics other than the punch through breakdown voltage cannot be defined in respect of the reliability and integration degree of the device characteristics. Furthermore, the prior art has a problem in that a device area cannot be reduced because the diode element is provided in addition to the bipolar element.
In consideration of adaptation to an increase in the fineness and performance of the device, moreover, the following problem arises. More specifically, a diffusion layer becomes shallow and the impurity concentrations of the base and the emitter are 10.times.10.sup.18 cm.sup.-3 or more and a Zener breakdown is caused in place of the conventional avalanche breakdown. Therefore, the reverse junction leakage current of the diode is increased so that the function of the diode cannot be performed.
On the other hand, Japanese Patent Application Laid-Open NO. Hei-8-146079 has disclosed a method for measuring a collector and emitter breakdown voltage of a bipolar transistor which is intended to avoid the influence of a snap back characteristic and to obtain the accurate measured value of a breakdown voltage without causing the destroy of a measuring element. In this method, a breakdown voltage BVCEO of an NPN bipolar transistor is measured by the following method. First of all, an emitter terminal is grounded. Next, a probe current of 100 .mu.A is caused to flow to a base terminal. Then, a decision current of 100 .mu.A is caused to flow to a collector terminal. The decision current should measure a punch through breakdown voltage BVCEO. A voltage at which the decision current flows is set to a breakdown voltage. Subsequently, a 0 current is caused to flow to the base terminal. The 0 current means that the base terminal is opened as described in an embodiment of the prior art official gazette (see paragraph 0019 and FIG. 1(b) of the official gazette). Consequently, although the decision current continuously flows, a base current approximates 0 and a voltage is raised. A voltage between the collector and emitter terminals is measured as BVCEO. In the prior art, thus, a bipolar operation is performed to cause the base current to flow, and the base current is then set to 0, thereby avoiding a negative resistive region to measure a breakdown voltage.
However, each junction area is decreased and a depth is reduced with the fineness of the device and a reduction in the size thereof in recent years. Under such circumstances, a constant current of 100 .mu.A is first caused to flow to a base in the prior art described in the Japanese Patent Application Laid-Open NO. Hei-8-146079. In this case, when the current of 100 .mu.A is instantaneously caused to flow, there is a particular possibility that the base and emitter junction might destroy. Therefore, it should take plenty of time to increase a current sequentially from 0 A. Furthermore, it is necessary to wait for the next operation until a current value is stabilized. For this reason, the above-mentioned prior art has had a problem in that a measuring time is increased and a throughput is deteriorated.